All regions of the U.S. are now experiencing influenza outbreaks, predominantly with the H1N1 "swine flu" strain, according to the Centers for Disease Control and Prevention (CDC). In contrast to some prior flu epidemics, young and middle-aged adults -- rather than young children and elderly people -- appear to have the highest risk of serious complications and death.

The World Health Organization (WHO) announced this week that 2009 H1N1 influenza A, popularly known as swine flu, has now entered a post-pandemic period. Although localized outbreaks are likely to continue, H1N1 is no longer being widely transmitted worldwide, as it was last fall and winter. Nevertheless, the agency stressed that the course of influenza pandemics is unpredictable and continued vigilance is warranted.

As a new flu season gets underway, a series of recent reports have looked at aspects of the 2009 H1N1 influenza strain responsible for last year's "swine flu" epidemic. Spanish researchers reported the promising finding that people with well-controlled HIV disease on antiretroviral therapy (ART) had H1N1 flu outcomes similar to those of HIV negative individuals. But another pair of studies found that HIV positive people -- especially those with low CD4 cell counts -- did not respond as well as HIV negative people to the H1N1 vaccine or an older flu vaccine. Finally, scientists with the National Institute of Allergy and Infectious Diseases (NIAID) recently projected what might happen with H1N1 this year and into the future.

H1N1 "swine flu" activity has fallen to a low level typically observed for seasonal influenza during the summer months in the U.S., according to the latest update from the Centers for Disease Control and Prevention (CDC). During the week of May 9-15, doctor visits for flu-like symptoms decreased, flu-related hospitalizations leveled off, and the number of deaths attributed to influenza or pneumonia remained stable. Globally, the most active areas of H1N1 flu transmission are in the tropical regions of the Caribbean, South America, and Southeast Asia.